The reduction-oxidation (redox) flow battery is an electrochemical storage device that stores energy in a chemical form and converts the stored chemical energy to an electrical form via spontaneous reverse redox reactions. The reaction in a flow battery is reversible, so conversely, the dispensed chemical energy can be restored by the application of an electrical current inducing the reversed redox reactions. A single redox flow battery cell generally includes a negative electrode, a membrane barrier, a positive electrode, and electrolytes containing electro-active materials. Multiple cells may be combined in series or parallel to create a higher voltage or current in a flow battery. Electrolytes are typically stored in external tanks and are pumped through both sides of the battery. When a charging current is applied, electrolytes lose electron(s) at the positive electrode and gain electron(s) at the negative electrode. The membrane barrier prevents the positive electrolyte and negative electrolyte from mixing while allowing ionic conductance. When a discharging current is applied, reverse redox reactions occur on the electrodes. The electrical potential difference across the battery is maintained by chemical redox reactions within the electrolytes and can induce a current through a conductor while the reactions are sustained. The amount of energy stored by a redox battery is limited by the amount of electro-active material available in electrolytes for discharge, depending on the total volume of electrolytes and the solubility of the electro-active materials.
Hybrid flow batteries are distinguished by the deposit of one or more of the electro-active materials as a solid layer on an electrode. Hybrid batteries may, for instance, include a chemical that plates as a solid on a substrate throughout the charge reaction and its discharged species may be dissolved by the electrolyte throughout discharge. In hybrid battery systems, the energy stored by the redox battery may be limited by the amount of metal plated during charge and may accordingly be determined by the efficiency of the plating system as well as the available volume and surface area to plate.
In a hybrid flow battery system the negative electrode may be referred to as the plating electrode and the positive electrode may be referred to as the redox electrode. The electrolyte within the plating side of the battery may be referred to as the plating electrolyte and the electrolyte on the redox side of the battery may be referred to as the redox electrolyte.
Anode refers to the electrode where electro-active material loses electrons. During charge, the negative electrode gains electrons and is therefore the cathode of the electrochemical reaction. During discharge, the negative electrode loses electrons and is therefore the anode of the reaction. Therefore, during charge, the plating electrolyte and plating electrode may be respectively referred to as the catholyte and cathode of the electrochemical reaction; the redox electrolyte and the redox electrode may be respectively referred to as the anolyte and anode of the electrochemical reaction. Alternatively, during discharge, the plating electrolyte and plating electrode may be respectively referred to as the anolyte and anode of the electrochemical reaction, the redox electrolyte and the redox electrode may be respectively referred to as the catholyte and cathode of the electrochemical reaction.
One example of a hybrid redox flow battery uses iron as an electrolyte for reactions wherein on the negative electrode Fe2+ receives two electrons and deposits as iron metal during charge and iron metal loses two electrons and re-dissolves as Fe2+ during discharge. On the positive electrode two Fe2+ lose two electrons to form two Fe3+ during charge and during discharge two Fe3+ gains two electrons to form two Fe2+:Fe2++2e−↔Fe0(Negative Electrode)2Fe2+↔2Fe3++2e−(Positive Electrode).
The electrolyte used for this reaction is readily available and can be produced at low costs (such as FeCl2). It also has a high reclamation value because the same electrolyte can be used for the plating electrolyte and the redox electrolyte, consequently eliminating the possibility of cross contamination. Unlike other compounds used in hybrid redox flow batteries, iron does not form dendrites during plating and thus offers stable electrode morphology. Further, iron redox flow batteries do not require the use of toxic raw materials and operate at a relatively neutral pH unlike similar redox flow battery electrolytes. Accordingly, it is the least environmentally hazardous of all current advanced battery systems in production.
The inventors have recognized various issues with the above system. Namely, under certain extreme charging conditions, such as low temperature, or high charging current (whereby Fe is rapidly produced at the negative electrode due to fast charging conditions), iron plating may be stressed and could crack and flake off the negative electrode. Higher plating stress can thus degrade the negative electrode and reduce the capacity and efficiency of the redox flow battery cell.
One approach that at least partially addresses the above issues is a method of operating an iron redox flow battery system comprising: fluidly coupling a plating electrode of an iron redox flow battery cell to a plating electrolyte; fluidly coupling a redox electrode of the iron redox flow battery cell to a redox electrolyte; fluidly coupling a ductile plating additive to one or both of the plating electrolyte and the redox electrolyte; and increasing an amount of the ductile plating additive to the plating electrolyte in response to an increase in the plating stress at the plating electrode.
In another embodiment, a method of operating an iron redox flow battery system may comprise, responsive to a charging current density applied to an iron redox flow battery (IFB) cell increasing above a threshold charging current density, raising a concentration of a ductile plating additive above a threshold concentration in a plating electrolyte fluidly coupled to a plating electrode.
In another embodiment, a redox flow battery system may comprise: a redox flow battery cell, including a plating compartment and a redox compartment; a plating electrode fluidly coupled to a plating electrolyte in the plating compartment; a redox electrode fluidly coupled to a redox electrolyte in the redox compartment; a ductile plating additive fluidly coupled to one or both of the plating electrolyte and the redox electrolyte; and a controller, including executable instructions to raise a concentration of the ductile plating additive in one or both of the plating compartment and the redox compartment in response to a charging current density increasing above a threshold charging current density.
In this way, ductile Fe can be plated on the negative electrode, and the performance, reliability and efficiency of the iron redox flow battery can be maintained. In addition, iron can be more rapidly produced and plated at the plating electrode, thereby achieving a higher charging rate for all iron flow batteries.